1. Field of the Invention
The present invention relates to a vibration control device which is suitable for use in a rotating machine such as a high-speed motor employed in textile machinery and the like.
2. Prior Art
FIG. 1 is a sectional view illustrating a mechanical structure of a high-speed motor which has a general construction of the motor conventionally known and is especially used in textile machine.
The motor shown in FIG. 1 is a so-called outer-rotor motor, in which a roller 2 having a hollow-cylinder-like shape rotates about a shaft 1 whose both sides are securely fixed. The roller 2 cooperates with the shaft 1 by means of bearings 3 at both sides thereof such that the roller 2 can freely rotate about the shaft 1.
A rotor core 5 is attached to an interior surface of the roller 2 through a magnet 4, while a stator core 6 is attached to the shaft 1 to face the rotor core 5 with an air gap therebetween. Upon the receipt of electric currents carried from a cable 7 which is introduced to the stator core 6 through a hollow portion of the shaft 1, a rotating magnetic field is produced around the stator core 6.
In short, the above-mentioned magnet 4, the rotor core 5 and the stator core 6 are assembled together to form a synchronous induction motor. Instead of the synchronous induction motor, it is possible to employ an induction motor. In the synchronous induction motor described above, the roller 2 rotates about the shaft 1. By pressing a hobbin winder against a peripheral face of the roller 2 which is rotating about the shaft 1, yarn is wound up.
Thus, the peripheral velocity of the rotating roller 2 is roughly proportional to the winding speed of the yarn.
It is desired to raise the winding speed of the yarn for an improvement of the productivity in the textile industries. For this reason, a higher peripheral velocity is required for the roller 2. For example, the peripheral velocity of 6,000 m/min! or so is required for the roller 2. In order to achieve such high peripheral velocity, there are provided two methods as follows:
1 a first method to enlarge an outer diameter of the roller 2; and
2 a second method to increase the number of revolution of the roller 2.
The first method raises a new problem in that the size of the roller 2 must be enlarged as well as another problem that a higher precision is required for the bearing 3 to respond to an increase of the weight of the roller 2.
Therefore, the second method is conventionally employed. In addition, in this method the diameter of the shaft 1 is reduced as well. Thus, it is possible to use a bearing which has a smaller maximum load and smaller major and minor diameters.
However, as the diameter of the shaft 1 becomes smaller, the frequency of natural vibration occurring on the shaft 1 should become lower. Under the effect of a large number of revolutions of the roller 2, the above-mentioned natural vibration should occur when the roller 2 rotates at a certain number of revolutions which is lower than the number of revolutions corresponding to the peripheral velocity to be required. This may adversely affect the operation of the motor.
Next, the natural vibration will be described in detail. In general, the natural vibration represents a mechanical characteristic which is inherently provided in the mechanical structure. When exciting the mechanical structure with its natural frequency, the resonance phenomenon occurs so that the mechanical structure may vibrate with a very large vibration.
Each of FIGS. 2A, 2B and 2C shows the manner of the natural vibration of the shaft 1 in connection with each of vibration modes. As compared to the shaft 1, the roller 2 has a greater rigidity. Hence, as compared to the shaft 1, the roller 2 has a higher natural frequency. Therefore, when observing the motor as a whole, the natural vibration of the roller 2 can be neglected. Thus, it is possible to study the manner of vibration of the motor by referring to only the natural vibration of the shaft 1.
It can be observed from FIG. 2A that no vibration is produced when the rotation of the roller 2 is stopped. However, when starting the rotation of the roller 2, due to the vibration accompanied with the rotation of the roller 2, the shaft 1 should be excited in vibration. Thereafter, when the number of revolution of the roller 2 reaches 7,980 rpm as shown in FIG. 2B, a certain natural vibration (at a frequency of 133 Hz) is produced on the shaft 1, both of whose securely fixed side edges act like nodes for the vibration. This manner of vibration will be represented by a term called "first-order vibration mode". In the case of the first-order vibration mode, the roller 2 as a whole should be largely vibrated up and down in accordance with the vibration of the shaft 1.
Thereafter, when the number of revolution of the roller 2 is further increased to reach 16,080 rpm as shown in FIG. 2C, a split natural vibration (at a frequency of 268 Hz) is produced on the shaft 1, in which both-side edges of the shaft 1 and a mid-point therebetween act like the nodes for the vibration. This manner of vibration will be represented by a term called "second-order vibration mode". In the case of the second-order vibration mode, due to the vibration of the shaft 1, the roller 2 should be vibrated such that the both-side edges thereof vibrate at different phases which are reverse (in phase opposition) to each other. In short, the roller 2 vibrates like a seesaw.
Similarly, every time the number of revolution of the roller 2 is increased by a certain number of revolutions, the split mode natural vibration is produced on the shaft i by using its both-side edges and other points as the nodes for the vibration. Each of the other points is located for each of equally divided parts which are disposed between both of the side edges of the shaft 1. Herein, the number of the equally divided parts is set as an integral number which is equal to or greater than "1". The manner of such high-order vibration will be represented by a term called "nth order vibration mode". Incidentally, as the integer "n" becomes higher, it becomes increasingly more difficult to neglect the natural vibration of an element other than the shaft 1.
When the natural vibration is produced on the shaft 1 as described above, the roller 2 is correspondingly vibrated. In a textile machine, such vibration is transmitted to the hobbin winder. This results in a lower quality of the yarn to be wound. Due to the natural vibration of the shaft 1, the rotor core 5 may come in contact with the stator core 6 (see FIG. 1), which causes a possibility that the electric motor itself will be damaged.
FIG. 3 is a sectional view illustrating the mechanical construction of a rotating machine 101 equipped with an overhang roller, which is applied to the textile machine used for spinning cotton into thread and the like. FIG. 3 is a view of an inner structure of the rotating machine 101 whose one-side portion from an axial line J is cut out. As shown in FIG. 3, a body of the rotating machine 101 is fixed with an object 102 which is to be driven by the rotating machine 101. A numeral 103 denotes a shaft which works as a rotating axis for the rotating machine 101. This shaft 103 is inserted through the body of the rotating machine 101 and is surrounded by a rotor 106. A stator 107 is further provided inside of the body of the rotating machine 101 such that the stator 107 surrounds a rotor 106. This stator 107 produces a magnetic field to produce rotation of the rotor 106.
One edge of the shaft 101 is supported by a bearing stand 141, provided at an edge portion of the body of the rotating machine 101, through a ball bearing B1. An intermediate portion and another edge portion of the shaft 101 supported by a bearing stand 142 through a ball bearing B2. A shaft 103 is protrudes toward the outside of the rotating machine 101 from the bearing stand 142. A roller 105 is attached to an edge portion of the shaft 103. The rotating machine 101 creates a rotation-driving force, which is transmitted through the roller 105 toward the object 102 to be driven.
In the above-mentioned rotating machine 101, under effects of the magnetic field produced by the stator 107, a rotating force is imparted to the rotor 106. This rotating force is transmitted to the roller 105 through the shaft 103. In the process of spinning cotton into thread and the like, the roller 105 which is driven to be rotated functions to impart some tension to the thread or it functions to guide the thread in a predetermined course.
In order to cope with the demand to improve productivity, the roller of the rotating machine tends to be enlarged in size. However, when enlarging the size of the roller, the axial-edge load and axial-edge mass applied to the rotating machine should be increased. This may cause a greater amount of imbalance in the rotating system containing the roller in the rotating machine. Based on the amount of imbalance, a greater vibration will be caused in the rotating machine. Such phenomenon of causing vibration due to the imbalance of the rotating system will be described in detail by referring to FIGS. 4 to 6.
In order to avoid the vibration of the rotating system, element causing the vibration should be removed from the construction of the rotating system, and the amount of imbalance of the rotation should also be eliminated. Actually, however, it is difficult to form each of the parts (e.g., rotor) of the rotating system perfectly in a axis-symmetrical shape. In other words, it is very difficult to arrange those parts in the rotating system perfectly in an axis-symmetrical manner. For this reason, a small deviation exits between the center of gravity and the rotation axis in the rotating system. When the rotating body in which the center of gravity deviates from the rotation axis is driven to be rotated, a vibration having a frequency which corresponds to rotation speed is inevitably caused in the rotating body. Other than the deviation of the center of gravity, there exists several kinds of elements which distribute to the amount of imbalance in the rotating system. Such amount of imbalance causes an excitation force in the rotating system, so that a certain vibration is excited in the rotating system.
FIG. 4 is a graph showing a characteristic of the vibration which is produced in the rotating system due to the above-mentioned amount of imbalance. In FIG. 4, characteristic curve A represents a relationship between a number of revolution "N" and an amplitude of the vibration at a point "a" of the roller 105 in the case where the rotating machine 101 is driven to be rotated. In general, the rotating system, which is constructed by the roller, shaft and the like, has a natural frequency of natural vibration. Under the state where the number of revolution of the rotating machine 101 is lower than the natural frequency, the rotating system is hardly affected by the excitation force which is caused due to the amount of imbalance. Therefore, as shown in FIG. 4, only a small vibration having a small amplitude is imparted to the rotor 105. Thus, it is possible to obtain a proper rotation in which an excessive bending stress is not imparted to the shaft as shown in FIG. 5.
However, when the number of revolution of the rotating machine 101 becomes closer to the natural frequency, the rotating system sensitively responds to the excitation force to be created due to the amount of imbalance, so that a large vibration should occur in the roller and the like. FIG. 4 indicates that the amplitude of the vibration at the point "a" of the roller 105 is raised to the maximum when the number of revolutions reaches a frequency N1 of first-order natural vibration (hereinafter, denoted to as a first-order natural frequency N1). Actually, there exist second-order and other higher-order natural vibrations; however, those higher-order natural vibrations are omitted in the graph of FIG. 4. When the above-mentioned large vibration occurs in the roller 105, a relatively big stress is imparted to the shaft 103. In the worst case, the shaft 103 is bent as shown in FIG. 6, which causes an extremely dangerous state for the rotating machine. The bending of the shaft 103 is somewhat exaggerated in FIG. 6 as compared to the actual bending.
In order to prevent the large vibration from being produced in the rotating system, a rated number of revolution "NMAX" of the rotating machine is generally set lower than the first-order natural frequency N1.
However, in order to increase the productivity, the rated number of revolution should be set higher. In order to do so, two countermeasures are required as follows:
1 to increase the first-order natural frequency; and
2 to reduce the amplitude of the vibration caused when the number of revolutions coincides with the natural frequency.
In order to increase the first-order natural frequency, the diameter of the shaft 103 should be enlarged. However, when enlarging the diameter of the shaft 103, a so-called "dn value" of the ball bearing supporting the shaft 103 must be also increased. However, this results in a reduction of the lifetime of the ball bearing. For this reason, there is a limitation to enlarging the diameter of the shaft.
Meanwhile, when the vibration having the first-order natural frequency N1 occurs in the roller, the amplitude of the vibration is determined by the amount of imbalance and a damping coefficient which is provided for the rotating system. Therefore, a degree of stability of the rotation can be raised by reducing the amount of imbalance; or the damping coefficient can be also increased. Actually, however, there is a limitation in doing this. In short, it is difficult to control (or reduce) the vibration of the roller at the first-order natural frequency.